Pyongyang's official Korean Central News Agency Monday said the move is a response to what the North calls "frequent intrusions by warmongers in the South Korean military."
North Korea has refused for decades to accept a maritime border known as the Northern Limit Line, mandated by the United Nations after a 1953 armistice halted fighting in the Korean War. North and South Korea fought two deadly naval battles in the area in 1999 and 2002, and the South badly damaged a North Korean ship in an exchange of fire there last month.
South Korea denies ever crossing into the North's side of the line. Seoul says every clash between the two sides has resulted from North Korean incursions into the South's waters.
Military experts here say under President Lee Myung-bak, South Korea has made its naval rules of engagement more robust, allowing ships to fire back at shore-based North Korean positions that fire on them first.
Yang Moo-jin is a North Korea scholar at Seoul's University of North Korean Studies. He says Monday's announcement is an attempt to put political pressure on South Korea and the United States.
He says North Korea is accentuating the instability of the Korean peninsula in a state of truce. He says the North is trying to convey to the U.S. and the international community the urgency of sealing a permanent peace treaty.
The prospect of such a treaty is written into a September 2005 agreement signed by North Korea. It says South Korea and the United States will work toward a peace treaty with the North gradually, as the North takes steps to get rid of its nuclear weapons. Pyongyang conducted its second nuclear weapons test earlier this year and has refused to return to talks on the issue.